Is there a term for a proposition emphasizing the impossibility of an event?
Propositions such as When Hell freezes over, When pigs fly, and When chickens get teeth are used to emphasize the impossibility of an event happening.
Is there a term for these expressions in the way that a terse statement of a general truth, such as “Power tends to corrupt, and absolute power corrupts absolutely," is an aphorism?
You may call them idioms of improbability
Wikipedia says:
There are many idioms of improbability, used to denote that something is impossible or unlikely to occur.